Method of making a micro-fluidic structure

ABSTRACT

A microfabricated structure and method of making that includes forming a first layer of material on a substrate, forming patterned sacrificial material having a predetermined shape on the first layer of material, and forming a second layer of material over the first layer and the patterned sacrificial material, which is then removed to form an encapsulated cavity. Ideally, the first and second layers are formed of the same type material. A structural support layer can be added to the second layer. Openings can be formed in the cavity, and the cavities can be layered side by side, vertically stacked with interconnections via the openings, and a combination of both can be used to construct stacked arrays with interconnections throughout.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to microfabricated structures and, moreparticularly, to the formation of above-substrate micro-fluidicstructures, such as cavities, enclosed chambers, and channels,preferably utilizing a single-type material.

2. Description of the Related Art

Micro-ElectroMechanical Systems (MEMS) refers to the fabrication andutilization of microscopic mechanical elements, such as sensors,actuators, and electronics, typically fabricated on or in silicon chipsor a silicon substrate using microfabrication technology. Thistechnology is borrowed from fabrication techniques used to formintegrated circuits (e.g., CMOS, bipolar, or BICMOS processes). MEMSdevices are generally mechanical components ranging in size from amicrometer (a millionth of a meter) to a millimeter (a thousandth of ameter), and can include three-dimensional lithographic featuresemploying various geometries.

Typical applications for MEMS devices and systems include piezoelectricsfor printers or bubble ejection of ink, accelerometers to control thedeployment of airbags, gyroscopes for dynamic stability control,pressure sensors used in transportation and medical applications, suchas car tire pressure sensors and disposable blood pressure sensors,micromirrors used to form displays, optical switching technology fordata communications, and heated chambers for fluidic applications.

A related technology is Nano-ElectroMechanical Systems (NEMS), which aresimilar to MEMS but on a smaller scale, including displacements andforces at the molecular and atomic scales. Together NEMS andnanotechnology have made it possible to provide mechanical andelectrical devices on a single chip that are much smaller, morefunctional and reliable, and produced at a fraction of the cost ofconventional macroscale elements. In many of these applications,chambers and channels are used for transporting, storing, manipulating,and sensing fluids both in gaseous and liquid form. The formation ofthese chambers and channels in MEMS devices presents unique fabricationchallenges.

Today, most fluidic chambers and channels in MEMS applications areconstructed from thick deposited materials in which the chamber orchannel is formed by either patterning and etching or by formation inthe substrate materials, such as the silicone substrate used to formintegrated electronic circuits.

One of the basic building blocks in MEMS microfabrication is the use ofthin-film deposition processes on a substrate, applying a patterned maskon top of the deposited film by photolithographic imaging, and etchingthe film utilizing a selective mask process.

Typical materials used are organic polymers, silicon, or variousglass-like films. Generally, the bottom, sides, and top surrounding thechannels are formed of three different material types for ease ofconstruction. Using fewer types of material increases the difficulty offabrication. Of the many available materials, the easiest to build with,organic polymers, have dimension control limitations because of thelarge shrinkage factor during curing (typically 25%). If not fullycured, they have poor adhesion characteristics and are not as resistantto the stresses of temperature and chemicals. While other materials areavailable with more desirable characteristics, they are impracticalbecause of the thickness required. In some cases, tens of microns ofvertical dimension are necessary in order to fabricate a fluidic chamberor channel.

FIG. 1 illustrates one type of conventional chamber structure 20 formedon a substrate 22. In this example, an optional integrated circuit 24 isformed on top of the substrate. A lower portion 26 of the chamber 28 isformed of a thin deposited film, while the chamber sidewalls 30 aretypically a thick organic “spin-on” material, such as polyimide, SU8 andFox. The top 32 of the chamber can be a rigid plate applied after thechannel is formed or a deposited material applied before the channel isformed. The disadvantage of this construction is, as alluded to above,the use of three separate materials, the thin film for the lower portion26, the spin-on material for the sidewalls 30, and the rigid materialfor the top plate 32. In addition to the aforementioned problems causedby the different materials, different processes are required, increasingthe complexity and cost of this structure.

FIG. 2 shows another approach to forming a channel or a chamberstructure 34 in which the substrate 36 is etched or otherwise excavatedto form the channel 38 that is then enclosed by a subsequent layer 40.In this case, only two materials are used, but the disadvantage remainsof using materials having potentially conflicting properties as well asthe necessity of using different processes.

BRIEF SUMMARY OF THE INVENTION

The disclosed embodiments of the present invention are directed to amicrofabricated structure and method of making same. It is to beunderstood that while the present invention will be described in thecontext of MEMS microfabrication techniques and applications, thepresent invention will have application to NEMS techniques andapplications as well as to other related technologies. Thus, whilerepresentative embodiments of the invention are described in the contextof MEMS technologies, the techniques can be more broadly applied.

In accordance with one embodiment of the invention, a method of forminga cavity is provided, the method including forming a first layer of afirst type of material on a substrate; forming patterned sacrificialmaterial on the first layer of material; forming a second layer ofmaterial on the first layer of material, preferably formed of the firsttype of material, and over the patterned sacrificial material toencapsulate the patterned sacrificial material within the first andsecond layers of material; and removing the patterned sacrificialmaterial to form the cavity enclosed within the first and second layersof material.

In accordance with another aspect of the foregoing embodiment, thecavity of the structure has a configuration that is in the shape of theremoved patterned sacrificial material. In addition, a structuralsupport layer can be formed on the second layer that is at leastadjacent to sides of the cavity.

In accordance with another embodiment of the invention, a method offorming a structure is provided, the method including forming amicrostructure, comprising: forming a first layer of a first type ofmaterial on a substrate; forming patterned sacrificial material on thefirst layer; forming a second layer of material on the first layer ofmaterial and over the patterned sacrificial material to encapsulate thepatterned sacrificial material within the first and second layers ofmaterial; and removing the patterned sacrificial material to form amicrostructure having an enclosed cavity.

In accordance with another aspect of the foregoing embodiment, thesecond layer is preferably of the same type of material as the firstlayer, and the cavity has a configuration that is in the shape of thepatterned sacrificial material removed from between the first and secondlayers.

In accordance with another aspect of the foregoing embodiment, astructural support layer is formed on the second layer that is at leastadjacent sides of the cavity to provide support for additional layers ofmaterial. This structural support layer can be formed of the same typeof material as the second layer or it may be formed of other materialthat is compatible with the second layer of material for its purposes.

In accordance with a further aspect of the foregoing embodiment, anopening is formed in the second layer that is in fluid communicationwith the cavity, and an opening is formed in the substrate and the firstlayer that is in fluid communication with the cavity. In one embodiment,the cavity is in the shape of a conduit having open ends to conductfluid therethrough.

In accordance with another embodiment of the invention, an integratedcircuit is formed on the substrate between the first layer and thesubstrate to form a microstructure. This microstructure, which includesthe openings described above, can be used to form a stacked compositestructure by stacking the microstructures on top of each other. Ideally,at least one microstructure is in fluid communication with at least oneother adjacent microstructure.

In accordance with another aspect of the foregoing embodiment, themicrostructures can be placed side by side to form a layer ofmicrostructures. When combined with the stacked composite structure, anarray of microstructures is formed.

In accordance with another aspect of the foregoing embodiment, thestructure can be formed without an integrated circuit between thesubstrate and the first layer, and a third layer of material is formedover the structural support layer and the second layer, and a fourthlayer of material is formed over the third layer of material and over apatterned sacrificial material placed on the third layer, and thepatterned sacrificial material between the third and fourth layers isremoved to form an enclosed cavity structure between the third andfourth layers.

In accordance with another aspect of the foregoing embodiment, openingsare formed in the third and fourth layers that are in fluidcommunication with the enclosed cavity between the third and fourthlayers and that is also in fluid communication with the cavity formedbetween the first and second layers.

In accordance with another embodiment of the invention, a structure isprovided, the structure including a first layer of material formed of afirst type of material on a substrate; a second layer of material formedover the first layer of material, the second layer of material formed ofthe same type of material as the first layer of material, the secondlayer of material forming a cavity between the second layer of materialand the first layer of material, the cavity having a configuration thatis in the shape of a patterned sacrificial material removed from betweenthe second layer of material and the first layer of material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The foregoing and other features and advantages of the present inventionwill be more readily appreciated as the same become better understoodfrom the following detailed description when taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 illustrates in cross-section a conventional fluidic channel orchamber structures;

FIG. 2 illustrates in cross-section an alternative approach to forming aconventional channel within a substrate;

FIGS. 3-5 illustrate the steps of the method in accordance with oneembodiment of the invention for forming a microfluidic structure;

FIG. 6 illustrates one alternative embodiment of a microfluidicstructure formed in accordance with the present invention;

FIG. 7 illustrates another embodiment of a microstructure formed inaccordance with the present invention;

FIG. 8 illustrates a further embodiment of a microstructure formed inaccordance with the present invention;

FIG. 9 illustrates stacked wafers and arrayed structures formed inaccordance with the present invention;

FIG. 10 illustrates stacked structures and arrays formed in accordancewith the present invention; and

FIG. 11 illustrates an alternative embodiment of a structure formed inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring next to FIGS. 3-5, illustrated therein is one embodiment ofthe invention that shows the steps of a method for forming a cavity. Asubstrate 42 is provided, preferably formed of silicon or other typicalmaterial used in forming microstructures or nanostructures. Thesubstrate 42 has a first surface, in this case a top surface 44 on whichis formed an optional layer of chip logic 46. It is to be understoodthat this chip logic can be formed on top of the substrate 42, or on topof and within the substrate 42, or solely within the substrate, as iswell known. It is also to be understood that the chip logic layer 46 isoptional and is not necessary to practice this embodiment of theinvention. The chip logic layer 46 has a top surface 48 as shown in theside view of FIG. 3 on which is formed a first layer 50 of a first typeof material. This first layer of material 50 will become the bottom of acavity to be formed. In one aspect of this embodiment, the first layerof material 50 can be a film formed of metals, metal oxides, oxides,nitrides, deposited silicon films, oxynitrides, siliconcarbide, or anarray of many materials. It can be deposited using one of many methodsknown to those skilled in the art, including chemical vapor deposition,electrodeposition, epitaxy, thermal oxidation, physical vapordeposition, and casting, among other methods.

The first layer 50 has an exposed surface 52 on which is formedpatterned sacrificial material 54 of a selected shape. Ideally, theselected shape of the patterned sacrificial material 54 is the spatialopposite of a final cavity to be formed. This material 54 is ideallyformed of “lost wax,” however other known sacrificial material can beused under appropriate conditions. The materials 54 has an exposed topsurface 56 and sidewalls 58 that are substantially orthogonal to theexposed surface 52 of the first layer 50 and to the top surface 56.

Referring next to FIG. 4, shown here is a subsequent step in which asecond layer of material 60 is formed over the first layer of material50 and over the patterned sacrificial material 54 to cover the topsurface 56 and sidewalls 58 of the patterned sacrificial material 54such that it is enclosed between the first layer 50 and the second layer60. Ideally, the second layer 60 functions as a liner for the subsequentchamber, channel, or cavity to be formed, and the second layer 60 isformed of the same type of material as the first layer 50 and, hence,the same process used to deposit or form the first layer 50 can be usedhere. At this point, the patterned sacrificial material 54 can beremoved or a structural support layer 64 can be formed over a topsurface 62 of the second layer 60 that extends to at least the portionsof the second layer 60 that cover the sides 58 of the patternedsacrificial material 54. In a preferred embodiment, the patternedsacrificial material 54 is first removed and then the structural supportlayer 64 is added. The structural support layer 64 is preferably appliedor formed by conventional methods on top of the second layer 60 andextends up to the level of the top of the second layer on the top sideof the cavity 66.

With the removal of the sacrificial material 54, a cavity 66 is thusformed between the first and second layers 50, 60 and can remaincompletely enclosed as shown in FIG. 5. The structure 70 shown in FIG. 5has the exposed surface 68 of the second layer 60 as the top surface ofthe structure 70. The particular structure 70 can be placed side by sidewith other structures 70, as shown in FIG. 6, to form a layeredmicrofabricated structure 72 having multiple cavities 66. Alternatively,the layered microfabricated structure 72 can be formed on a unifiedsubstrate 42 or on separate substrates that are then placed together.Hence, the layered structure 72 can be formed of segments ofmicrofabricated structure 70 or as a single unitary structure in whichthe layers are deposited across the entire substrate 42.

FIG. 7 illustrates another embodiment of the invention in which themicrofabricated structure 70 has an opening 74 formed in the exposedsurface 68 and through the second layer 60 to be in fluid communicationwith the cavity 66. In one embodiment, the cavity 66 can be formed as anelongate conduit having at least one and preferably two or more openings74 to enable fluid, such as a gas or a liquid, to be conducted throughthe cavity 66. Microvalving may be used to control fluid flow into andout of the cavity.

FIG. 8 illustrates yet another embodiment of the invention in which achannel 76 is formed in the substrate and a subsequent opening 78 isformed through the chip logic layer 46 and the first layer 50 to be influid communication with the cavity 66. A combination of the embodimentsshown in FIGS. 7 and 8 may also be formed, which is shown in FIG. 9.Here, a stacked composite structure 80 is constructed from a combinationof the microfabricated structures 71 and 73 in which the microfabricatedstructure 73 having the triangular-shaped channel 76 formed in thesubstrate 42 is placed over the microfabricated structure 71 to be influid communication with the opening 74 and the cavity 66 thereof. Thisstacked composite structure 80 can be placed side by side with anotherstacked composite structure 80 to form an array structure 82.

Referring next to FIG. 10, shown therein is still yet another embodimentof the invention in which the structure 71 from FIG. 7 having theopening 74 in the top surface 68 has additional layers 84 of cavities 66formed thereon. More particularly, a third layer of material 86 isformed over the second layer of material 60 and the structural supportlayer 64, after which patterned sacrificial material is placed over thethird layer as described above with respect to FIGS. 3-5. A fourth layer88 of material is formed over the third layer 86 and the patternedsacrificial material, and the patterned sacrificial material is thenremoved to form the cavity 66, as described above with respect to FIGS.3-5. Ideally the first, second, third, and fourth layers are all formedof the same type material. Each cavity layer 84 has openings 90 formedtherein through the third layer 86 and the fourth layer 88 to enablefluid communication between the layers of cavities 66. Here, thisstacked cavity structure 92 can be placed adjacent another stacked arraystructure 92 to form a composite array structure 94.

Thus, the embodiments shown in FIGS. 7 and 8 show possible openings intothe cavity 66 for fluid inlet and/or outlet, visual inspections, fluidpressure control, or any of several applications. The cavities 66 may bestacked in vertical orientation with interconnections via the openings74, 76, 78, and 90 in any location. With the various arrays asillustrated in FIGS. 9 and 10, a honeycomb effect can be achieved withinterconnections throughout.

While preferred embodiments of the invention have been illustrated anddescribed, it is to be understood that various changes can be madetherein without departing from the spirit and scope of the invention.For example, FIG. 11 shows the structure 70 from FIG. 5 having thestructural support layer 64 extending further above the cavity 66 andthe second layer 60. This provides additional topside support for thecavity 66, especially when additional structures are to be stacked,formed, or deposited thereon. Hence, the invention is to be limited onlyby the accompanying claims and the equivalents thereof.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A method of forming a cavity on a substrate, comprising: forming afirst layer of a first type of material on a substrate; formingpatterned sacrificial material on the first layer of material, thepatterned sacrificial material formed to have sidewalls and a topsurface; forming a second layer of the first type of material on thefirst layer of material and over the patterned sacrificial material toencapsulate the patterned sacrificial material within the first andsecond layers of material; removing the patterned sacrificial materialto form the cavity enclosed within the first and second layers ofmaterial; and adding structural support material on the second layer ofmaterial that abuts the second layer adjacent the sidewalls of thepatterned sacrificial material and extending only to a level of a ton ofthe second layer on a top side of the cavity.
 2. The method of claim 1,further comprising forming an opening in the second layer of materialthat is in communication with the cavity.
 3. The method of claim 1,further comprising forming openings in the second layer of material thatare in fluid communication with the cavity to enable movement of afluidic substance into and out of the cavity.
 4. A method of forming astructure, comprising: forming a first microstructure, comprising:forming a first layer of a first type of material on a substrate;forming patterned sacrificial material on the first layer to have sidesand a top surface; forming a second layer of material on the first layerof material and over the patterned sacrificial material to encapsulatethe patterned sacrificial material within the first and second layers ofmaterial; forming a layer of structural support material on the secondlayer adjacent the sides of the patterned sacrificial material andextending to a level of a top of the second layer on the top surface ofthe patterned sacrificial layer; and removing the patterned sacrificialmaterial to form a microstructure with an enclosed cavity.
 5. The methodof claim 4 wherein the second layer comprises material of the same typeas the first layer.
 6. The method of claim 5 wherein the cavity has aninternal configuration in the shape of the patterned sacrificialmaterial.
 7. The method of claim 6 wherein the structural supportmaterial extends above the top of the second layer on the top surface ofthe patterned sacrificial layer to form a substantially planar supportsurface.
 8. The method of claim 7, comprising forming a secondmicrostructure on top of the first microstructure that is supported bythe substantially planar support surface.
 9. The method of claim 5,further comprising, before the step of forming a microstructure, formingan integrated circuit in association with the substrate and over whichthe first layer of material is formed.
 10. The method of claim 5,further comprising forming the cavity with openings to enable fluid toflow into and out of the cavity.
 11. The method of claim 5, furthercomprising forming a plurality of microstructures in a side-by-siderelationship.
 12. The method of claim 5, further comprising forming anopening in the second layer that is in fluid communication with thecavity, and forming an opening in the substrate and the first layer thatis in communication with the cavity.
 13. The method of claim 12, furthercomprising forming a stacked composite structure by stacking themicrostructures.
 14. The method of claim 13 wherein each microstructurein the stacked composite structure is in fluid communication with atleast one other adjacent microstructure.